This invention relates to a heat exchanger of vertical type particularly adapted to prevent backward flow of a fluid, so-called recirculation, of high temperature delivered from an inlet side plenum to an outlet side plenum in response to variation of the static pressure in the outlet side chamber.
Heat exchangers have been widely used for heating and cooling fluid, and for extracting and yielding vaporized or condensed substances in chemical plants, petroleum chemical plants, power generation plants or like plants.
Although an example of application of a vaporizer as one kind of a heat exchanger adapted to extract vaporized substance or material is disclosed hereinbelow, it should be noted that the term "heat exchanger" used herein totally includes a heater, a cooler, a vaporizer and a condenser of the character described above.
A vaporizer, for example a vertical type naphtha vaporizer in a fuel cell power plant, has a construction as shown in FIG. 5.
Referring to FIG. 5, naphtha in the liquid state introduced into a shell 2 of a drum shape through a liquid naphtha inlet tube 1 enters into a header 3, and is then atomized into fine particles through spray nozzles 4 into an inlet plenum 5. A mixing gas essentially consisting of hydrogen gas and carbon dioxide gas is introduced through a mixing gas inlet 6 into the inlet side plenum 5 in which the mixing gas is mixed with the atomized naphtha. The mixed gas is thereafter delivered to heat exchanger or heat transfer tubes 7 which extend in parallel through a heat exchanging chamber 7a, each of which is supported at both ends by tube plates 12a and 12b, respectively, and at its intermediate portion by baffle plates 13a and 13b secured to the inner wall of the shell 2.
The mixed gas delivered into the heat exchanger tubes 7 flows therethrough and is heated through heat exchanging operation performed between the mixed gas in the heat exchanger tubes 7 and heating gas introduced into the shell 2 through a heating gas inlet 10 and flowing along the outer surfaces of the heat exchanger tubes 7. Due to the heat exchanging operation, the naphtha in the mixed gas is vaporized and flows as a superheated gas towards an outlet side plenum 8 which is located in the shell 2 at a position opposite to the inlet side plenum 5. The mixed gas is thereafter delivered as a fuel gas into a reactor, not shown, through a fuel gas outlet 9 provided at the wall of the outlet side plenum 8, and then is delivered into a reforming device, not shown.
The heating gas introduced into the shell 2 flows meanderingly along the outer surface of the respective heat exchanger tubes 7. During this process, the temperature of the heating gas is reduced by heat exchanging between the heating gas and the mixed gas in the heat exchanger tubes 7. The heating gas is finally discharged to the outside of the shell 2 through a heating gas outlet 11. It is of course desirable that the heating gas inlet 10 is disposed near the tube plate 12a and the heating gas outlet 11 is disposed near the tube plate 12b.
In the naphtha vaporizer of the character or construction described above, however, the backward flow, so-called recirculation, of the heated fuel gas from the outlet side plenum 8 towards the inlet side plenum 5 through the heat exchanger tubes 7 in the direction reverse to the ordinary flow direction thereof may be caused in a case where the naphtha atomized by the spray nozzles 4 is delivered into the heat exchanger tubes 7 together with the mixing gas with a non-uniformly distributed amount. Accordingly, the constant flow rate of liquid state naphtha through the respective heat exchanger tubes 7 cannot be maintained.
In general, when an average flow rate of the liquid state naphtha is ensured, the static head thereof can be maintained and the inner pressure in the outlet side plenum 8 is kept higher than that in the inlet side plenum 5, whereas when the flow rate of the liquid state naphtha is largely below the average flow rate thereof, the static head is also lowered and the pressure relationship between the plenum 5 and 8 may be reversed through the heat exchanger tubes 7. In such an adverse state, the heated fuel gas in the outlet side plenum 8 flows reversely therefrom towards the inlet side plenum 5, thus starting the recirculation of the heated fuel gas.
According to a mock-up test to examine the possibility of the occurrence of the recirculation, it was observed that the liquid state naphtha having a flow rate about twice as much as the average flow rate flows in a certain heat exchanger tube 7 having the maximum flow rate, and the liquid state naphtha having a flow rate about half as much as the average flow rate flows in a certain heat exchanger tube 7 having the minimum flow rate. It is assumed from the result of this mock-up test that the heated fuel gas must flow reversely in the heat exchanger tube having the minimum flow rate of about half of the average flow rate.
Upon occurrence of such recirculation phenomenon of the heated fuel gas, the tube plate 12a of the inlet side plenum 5 and the heat exchanger tubes 7 may suffer from rapid temperature variation and extremely large thermal stresses will be generated. The repetition of such adverse phenomenon may finally result in breakages of these members due to accumulated fatigue.